Use of output voltage feedback arrangements has become popular for control of power converters and voltage regulators, particularly for point of load (POL) power supplies and voltage regulators for microprocessors where supplied voltage must be constrained within close tolerance even though load current may vary widely and rapidly. Some integrated circuits require not only regulation of voltage within close tolerances but also specify a reduction of voltage with increasing current, referred to as voltage droop, in order to better manage heat dissipation caused by extreme integration density and high clock rates. In any case, voltage feedback in a switching voltage regulator or power converter is usually performed by feeding the output voltage converter back to a comparator that compares the output voltage to a reference voltage and produces a pulse width modulated (PWM) control signal for the switches of the switching regulator or converter. This type of control can be enhanced by providing an additional integrating feedback loop such that the output voltage is a function of both the average and instantaneous output voltage and which is referred to as V2 control.
It has been shown that output voltage feedback for voltage regulation is essentially a current mode control with load current feedback and that the load current feedback and the inductor current feedback share the same load sensing gain, Rco, since the capacitor current is the difference between the inductor current and the load current. While the output or load current can be sensed in numerous ways, using the equivalent series resistance (ESR) of the output capacitor achieves fast line and load transient response in a simple manner since the ESR of the output capacitor is the same as the current sensing gain.
However, types of capacitors that have sufficient ESR for current sensing such as so-called OSCON™ or POSCAP™ capacitors are of large size and relatively short usable lifetimes. For these reasons, so-called ceramic capacitors (which generally but not necessarily include a component of ceramic material) are generally preferred in commercial power management products for their characteristically small size and long lifetime although use of ceramic capacitors as the output capacitor of a V2 controlled converter causes instability (referred to as sub-harmonic instability) at practical switching frequencies within several MHz. The small signal model is provided in “Modeling of V2 Current-Mode Control” by Jian Li et al., Circuits and systems I: Regular Papers, IEEE Transactions on, Vol. 57, No. 9, pp. 2552-2563, September, 2010. Additionally, even when the switching frequency is greater than several MHz, and the system is theoretically stable, jittering (because the ripple magnitude is small and decreases with increasing switching frequency compared with inevitable noise) is unacceptable. Several approaches to stabilizing or compensating voltage feedback control, and V2 control, in particular, using ceramic capacitors have been proposed but each has presented additional problems that limit the application for which the power converter may or may not be appropriate.